söndag 23 september 2012

Unphysical AIAA Lift Theory

Performance, Stability, Dynamics and Control by B. N. Pamadi in AIAA Education Series, describes the generation of lift of an airfoil as follows (with illustrations above):
  • The lifting line theory will be used to obtain expressions for the lift-curve slope.
  • In lifting line theory the wing is modeled as a horse-shoe vortex formed by a bound vortex, trailing vortices from the wing tips completed by a so-called starting vortex.
  • The bound vortex induces an upwash in from of the wing and a downwash behind. The trailing vortices induce downwash everywhere ... negligible in the vicinity of the wing.
  • To understand how a starting vortex can be a physical reality....
This is Prandtl's lifting line theory. The basic idea is that the bound vortex generates upwash in front and downwash behind and thus generates lift.

As indicated by the phrase "to obtain expressions" (for the lift-curce slope), lifting line theory is on one hand not really supposed to describe actual aerodynamics but only give a way of computing the one number of the lift-curve slope, and on the other hand it is described in suggestive pictures as if being real in some sense, although the action of the starting vortex is surrounded by mystery. The presentation is thus ambigious.

The truth is that lifting line theory does not describe actual physics: The upwash in front of the wing is not seen in real flow, nor in Navier-Stokes simulations, and in fact not in the top left picture either. What is seen is downwash at the trailing edge as a result of 3d rotational separation as described in the New Theory of Flight.

The New Theory is in the words of AIAA editor Greg Blaisdell, when rejecting it for publication in AIAA Journal, described as follows:
  • Your paper is unusual in that it challenges our existing understanding of aerodynamics.
Yes, it does, and shows that existing understanding is incomplete. To reject work increasing understanding is not beneficial to the science of aerodynamics and to engineers designing airplanes.

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